Various processes exist for welding plastic molded parts, see for example, Kunststoffe 87, (1997), 11, 1632-1640. A prerequisite for a stable weld joint in heated element welding and vibration welding processes, such as used for vehicle manifold pipes, is a sufficient softening of the joining partners in the contact zone before the actual joining stage.
When pressing welding partners together, a decisive factor for the strength of the weld joint to be formed is that the applied pressure lies in a specific optimum range. The pressure should not be too great because too much melt can be forced out from the contact zone, but on the other hand, should not be too low because weak points can occur in the weld joint. This is due, inter alia, to the fact that with many technical parts that are welded to one another, a 100% fitting accuracy cannot be achieved. Although contact of the molded part halves, over the whole weld joint, can be achieved by applying a sufficiently high compressive force, the local different pressure conditions may lead to a variously large outflow of melt from the weld joint and thus to locally variously high strengths. The problem can be alleviated by, for example, increasing the melt viscosity, as disclosed in EP-A 0 685 528, resulting in a reduced outflow of the melt from the joining zone.
As an alternative method to vibration welding and heated element welding, laser beam welding, in particular with diode lasers, has recently become increasingly widespread. The basic principle of laser beam welding of plastics is the absorption of radiation in the molding composition. Pure polymers are largely transparent or translucent to laser radiation, i.e. they hardly absorb laser light in the wavelength range from 700 to 1200 nm, which is of interest in laser beam welding. The absorption and thus the conversion of laser light into heat can be controlled by the use of pigments, fillers, reinforcing substances and additives.
The basic principles of laser beam welding are described in the literature, see for example, Kunststoffe 87 (1997) 3, 348-350; Kunststoffe 88 (1998) 2, 210-212; Kunststoffe 87 (1997) 11, 1632-1640; Plastverarbeiter 50 (1999) 4, 18-19; and Plastverarbeiter 46 (1995) 9, 42-46.
A precondition for the use of laser beam welding is that the radiation emitted by the laser first passes through a joining partner that is sufficiently transparent to laser light at the wavelength that is employed, and is then absorbed by the second joining partner in a thin layer of several 100 μm thick and is converted into heat that leads to the melting of the two joining partners in the contact zone and finally to the joining by a weld joint.
Amorphous polymers, such as, for example polycarbonates (PC), polystyrene (PS), poly(methyl methacrylate) (PMMA), as well as, partially crystalline thermoplastics materials such as, polyamides, polyamide 6 (PA6) and polyamide 66 (PA66), or polyesters, such as polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) are largely transparent or laser-translucent in the wavelength range of the lasers that are normally used for thermoplastics welding (Nd:YAG-Laser: 1060 nm; high output diode laser: 800-1000 nm).
Where the expressions laser-transparent, laser-translucent and laser absorbing are used hereinafter, they refer to the wavelength range mentioned above. However, in the case where fillers or reinforcing substances are used with partially crystalline materials there is a more or less marked scattering of the laser light due to the partially crystalline morphology, therefore the diffuse transmission accounts for a large part of the overall transmission. The transparency falls with increasing diffuse transmission, and the corresponding test specimens or molded parts should then be described more accurately as laser-translucent. Molding compositions that are only laser-translucent will hereinafter also be described as laser-transparent molding compositions. The measurement of the transmission capacity for IR laser light is usually performed with the spectrophotometer and an integrating photometer bulb. This measurement arrangement also detects the diffuse portion of the transmitted radiation. The transmission capacity is measured not only at one wavelength, but in a spectral range that covers all laser wavelengths that are currently used for laser beam welding.
The transmission of laser light is impaired by constituents of the thermoplastic molding compositions that absorb the light of the corresponding wavelength. These are in particular carbon blacks, but also other dyes, pigments or fillers and reinforced substances, e.g. carbon fibers. For a successful technical use of laser beam welding for joining molded parts of thermoplastic molding compositions, a molding composition that absorbs laser radiation and a molding composition that is largely transparent to laser radiation is therefore preferred.
For laser-absorbing molding compositions, carbon black-containing molding compositions are used, for example, in the case of glass fiber-reinforced PA6 compounds. Such molding compositions are for example conventionally also used for manifold pipes in vehicle internal combustion engines that are joined by vibration welding or by heated element welding, see Kunststoffe 91, (2001), 7, 100-102.
If the absorption of the radiation takes place very near the surface, which in certain circumstances leads to a reduced weld joint strength, an improvement may possibly be achieved by reducing the carbon black concentration, see Kunststoffe 87, (1997), 3, 348-350.
EP-B 0 751 865 describes a process for laser beam welding in which a first workpiece part, having a transmission of greater than 60%, is welded to a second workpiece part, having a negligible transmission. The low transmission of the housing base is achieved by pigmenting with dyes in an amount of 1% to 2%, and for the housing cover, is achieved with a lower coloring agent concentration, possibly also without the use of pigments. The specification does not describe which pigments or coloring agents are suitable.
DE-A 199 60 104 describes how two or more dark-colored molded parts can be joined to one another by laser beam welding. For example, dark to black molded parts that are colored with combinations of coloring agents that do not absorb in the near IR region of the laser welding are welded to molded parts containing coloring agents, in the simplest case carbon black, that absorb in the near IR region of the laser welding.
EP-A 1 029 650 describes the use of laser-transparent polyester molding compositions that are colored black by combinations of yellow and violet coloring agents, for laser beam welding with laser-absorbing polyester molding compositions.
The use of laser beam welding for the production of containers with cylindrical or similar cross-sections is described in DE-A 19 916 786. WO-A 2000/66346 describes the use of laser beam welding in the production of, inter alia, synthetic carpets, while further applications include welded tubing, as described in DE-A 44 25 861, or the joining of sheets or films, as described in JP-A 11170371.
In principle, the combination of, for example, non-colored glass fiber-reinforced molding compositions as a laser-transparent partner with a molding composition based on the same polymer colored by means of carbon black as laser-absorbing partner is therefore one possible way of producing a solid joint between two molded parts.
The use of black thermoplastic molding compositions, colored with carbon black as the laser-absorbing partner in laser beam welding is however, not possible for many applications. This relates to applications in which a certain transparency or translucency of the corresponding molded parts is necessary. For such applications as containers for servomechanism oil, brake fluid, coolants, etc., non-colored thermoplastics materials are generally used. However, the non-colored thermoplastic materials cannot be employed as laser-absorbing welding partners in laser beam welding due to their laser translucency. Also, the use of carbon black as the “laser absorber” is generally not possible in light colorations. Although in this case no transparency or translucency of the molded parts is desired, nevertheless there should as far as possible be no or only slight color differences between the joining partners.
One suggested solution for the welding of transparent light-colored molding compositions without change in color is disclosed in DE-A 19925203, which discloses the use of a welding additive from which a laser-absorbing substance is produced only by the application of, for example, electrical or thermal energy, which substance can then be converted again into the original laser-transparent substance during the welding process. The welding additive may be present as a film, sheet or wire in the joining zone or may be homogeneously distributed in the molded part that is to act as the absorbing joining partner. Although this process meets the object of achieving a discoloration-free welding of transparent joining partners, it is relatively complicated and potentially trouble-prone due to the additional process step involved in the production of the absorber before the welding. Specific examples that could demonstrate the applicability of the process to the welding of thermoplastics materials such as PA, PBT or PC are not given.
Combinations of joining partners that are pigmented in various colors represent a further special case. Here too a partner that is transparent to laser radiation and a partner that absorbs laser radiation are required. Thermoplastics materials that are colored with conventional coloring agents generally absorb only in one part of the visible spectrum and IR region of the spectrum however, with the result that they are generally not suitable or are only of limited suitability as laser-absorbing joining partners for laser beam welding.
The most commonly used coloring agent for coloring technical thermoplastics materials black is carbon black, in which carbon blacks are used that are produced by various processes and have different grain size distributions or specific surfaces. The use of carbon blacks for coloring materials black is significantly less expensive than organic or inorganic coloring agents.
However, in many cases coloring with carbon blacks or inorganic pigments has a negative effect on mechanical properties of technical thermoplastics materials, in particular on the toughness, measured for example as the Izod impact toughness according to ISO 180 1U. Here there is a need for laser-absorbing additives, in particular for those that enable the use of carbon black to be avoided completely or at least substantially, and that permit dark to black colorations with less impairment of the mechanical properties.
In many technical thermoplastics materials, e.g. in polyamide 6 and polyamide 66, carbon blacks also act as nucleating agents, i.e. the carbon black acts as a crystallization seed in the polyamide melt and thus promotes crystallization. Accelerated crystallization frequently leads to a deterioration of the surface quality, in particular in molded parts produced by the injection molding process. For this reason the lowest possible concentration of carbon black is often employed in thermoplastics molding compositions.
Therefore, the unrestricted coloring of the joining partners to be welded together by means of laser beam welding while at the same time preserving the best possible surface quality is desirable.